1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method and apparatus for performing cell reselection in a wireless communication system.
2. Related Art
A global system for mobile communication (GSM) is a radio technology which has been developed as a system for standardizing radio communication systems in Europe. A general packet radio service (GPRS) is a technique introduced to provide a packet switched data service in a circuit switched data service provided from the GSM. A universal mobile telecommunication system (UMTS) is a wireless communication system based on wideband code division multiple access (WCDMA). E-UTRAN is a wireless communication system based on orthogonal frequency division multiple access (OFDMA).
To overcome a restriction of a service area and a limitation of a user capacity, the wireless communication system provides a communication service by dividing the service area into a plurality of cells. This is referred to as a multi-cell environment.
Cell selection is a process in which a mobile station (MS) selects a cell to receive a service. The purpose of the cell selection is to register the MS to a network in order to receive the service from a base station (BS). In general, the MS selects a cell having a strongest signal property value by performing a signal measurement process with respect to the BS in all searchable frequency bands. The cell selected by the MS is referred to as a serving cell. A cell neighboring to the serving cell is referred to as a neighbor cell.
As the MS moves, it may be inappropriate to receive the service from the serving cell. In this case, the MS reselects a suitable cell capable of maintaining service quality and receiving a signal with better quality. This is referred to as cell reselection.
The cell reselection is a process of selecting a more suitable cell based on criteria for cell reselection and signal measurement in an idle state or a packet transfer mode. In the idle state, the MS repeats a process of reselecting a cell having a better signal property by measuring signals in the serving cell and the neighbor cell. For the MS with mobility, the cell reselection is important in order to receive a service with better quality.
Heterogeneous networks using various types of radio access technology (RAT) are introduced. The RAT is a type of technology used for radio access. Examples of the RAT include GSM/EDGE radio access network (GERAN), a UMTS terrestrial radio access network (UTRAN), an evolved-UMTS terrestrial radio access network (E-UTRAN), and so on. The GERAN, the UTRAN, and/or the E-UTRAN coexist in the same area. The E-UTRAN is also called long term evolution (LTE).
Hereinafter, a GSM cell (or a GERAN cell) is a cell using the GSM/GPRS (or GERAN) as the RAT. A UTRAN cell is a cell using the UTRAN as the RAT. An E-UTRAN cell is a cell using the E-UTRAN as the RAT.
The introduction of various types of RATs results in a problem of the interoperation between RATs. For the interoperation between different RATs, the MS may support multi-RAT. The MS supporting the multi-RAT may reselect a cell having another RAT different from the RAT of the serving cell. This is called inter-RAT cell reselection. For example, if the serving cell is a GSM cell, the MS may reselect a neighbor UTRAN cell or E-UTRAN cell.
For inter-RAT cell reselection, priority-based inter-RAT cell reselection is disclosed in the section 6.6.6 of 3GPP TS 45.008 V9.1.0 (2009-11) “Technical Specification Group GSM/EDGE Radio Access Network; Radio subsystem link control (Release 9)”. The MS attempts to camp on a cell having a frequency of an RAT having a top priority.
Accordingly, there is a need for a method capable of effectively performing a cell reselection process by an MS in heterogeneous networks in which various types of RATs coexist.